Update ci.yml

[fxamacker]

Description

• Bump actions/checkout to v3.
• Bump actions/setup-go to v3.
• Add Go 1.18, remove Go 1.16 (test latest 3 versions).

fixes: https://github.com/onflow/atree/issues/230

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• [x] Targeted PR against main branch
• [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• [x] Code follows the standards mentioned here
• [x] Updated relevant documentation
• [x] Re-reviewed Files changed in the Github PR explorer
• [x] Added appropriate labels

[codecov-commenter]

Codecov Report

Merging #252 (e17fa83) into main (5494f9e) will decrease coverage by 0.06%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #252      +/-   ##
==========================================
- Coverage   68.97%   68.90%   -0.07%     
==========================================
  Files          14       14              
  Lines        7245     7245              
==========================================
- Hits         4997     4992       -5     
- Misses       1617     1621       +4     
- Partials      631      632       +1     

Impacted Files	Coverage Δ
storage.go	79.28% <0.00%> (-0.36%)	:arrow_down:
map.go	71.31% <0.00%> (-0.11%)	:arrow_down:

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[turbolent]

Nice!

Maybe we can also bump https://github.com/onflow/atree/blob/main/go.mod#L3, e.g. to 1.17? See https://go.dev/doc/modules/gomod-ref#go-notes

(Once bumped, also need to run go mod tidy to update the go.sum file)